This application claims benefit of priority to Japanese Application No. JP 2001-279981 filed Sep. 14, 2001 and JP 2001-329405 filed Oct. 26, 2001, the entire content of which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to an economic, but high-efficiency power conversion device consisting of a combination of power diode rectifiers and voltage-type self-commutated power converters and/or a combination of power diode rectifiers and multi-level output self-commutated power converters.
2. Description of the Related Art
For electric railway power generation systems, a system is often adopted in which three-phase AC power is converted to DC power by power diode rectifiers in a three-phase bridge connection. This system has the advantages of excellent ability to withstand overloading and that the converter cost can be kept low. However, there was a problem that when regenerative braking was applied to the vehicle, the power involved in this could not be regenerated on the AC power source side, resulting in repeated absence of regeneration. Another drawback was load current dependence, resulting in considerable fluctuations of the DC generated voltage depending on the load.
FIG. 1A and FIG. 1B illustrate the circuit layout of a prior art PWM converter (pulse width modulation control converter) capable of power regeneration. In PWM converter CNV, the AC terminals are connected with terminals R, S, T of a three-phase AC power source SUP through an AC reactor Ls and the DC terminals are connected with the DC terminals of a DC smoothing capacitor Cd and three-phase output VVVF (variable voltage variable frequency) converter INV. The AC terminal of converter INV is connected with an AC motor M. The PWM converter CNV comprises six arms i.e. six rectifying high-speed diodes D1 to D6 connected in the form of a three-phase bridge and self-turn-off elements S1 to S6 consisting of a switching element for a regenerating inverter connected in anti-parallel with these diodes. Diodes D1 to D3 and self-turn-off elements S1 to S3 are arranged on the positive side and diodes D4 to D6 and self-turn-off elements S4 to S6 are arranged on the negative side. Inverter INV also has the same as circuit layout as converter CNV, so detailed description thereof is here omitted.
PWM converter CNV is equipped with a control device comprising comparators C1, C3, voltage control compensator Gv(S), multiplier ML, current control compensators Gi(S) and pulse width modulation control circuit PWMC. Comparator C1 and voltage control compensator Gv(S) are common to each phase but multiplier ML, comparator C3, current control compensators Gi(S) and pulse width modulation control circuit PWMC are provided for each phase. Only the internal circuit layout of the R phase is described in detail herein, but the layout of the S phase and T phase control circuits is the same. Gate signals g1, g4 for the self-turn-off elements S1, S4 of the R phase are output from the R phase control circuit; gate signals g2, g5 for the self-turn-off elements S2, S5 of the S phase are output from the S phase control circuit; and gate signals g3, g6 for the self-turn-off elements S3, S6 of the T phase are output from the T phase control circuit.
PWM converter CNV uses the control circuit constructed as above to control the input currents Ir, Is, It such that the DC voltage Vd applied to the DC smoothing capacitor Cd matches the voltage instruction value Vd*. In more detail, the deviation between the voltage instruction value Vd* and the voltage detection value Vd is obtained by comparator C1 and amplified by voltage control compensator Gv(S) and is taken as the amplitude instruction value Ism of the input current. Multiplier ML multiplies the amplitude instruction value Ism of the input current with a unit sine wave sin xcfx89t synchronized with the voltage of the R phase and this product is taken as the current instruction value Ir* of the R phase. Comparator C3 compares this R phase current instruction value Ir* with the R phase current detection value Ir and the deviation is subjected to inverse amplification by current control compensator Gi(S). Normally proportional amplification is employed, the gain being Gi(S)=xe2x88x92Ki, where Ki is the constant of proportionality (proportional constant).
The voltage instruction value er*=xe2x88x92Ki X(Ir*xe2x88x92Ir), which is the output of current control compensator Gi(S) is input to PWM control circuit PWMC to create the gate signals g1, g4 of self-turn-off elements S1 and S4 of the R phase of converter CNV. PWM converter circuit PWMC compares the voltage instruction value er* and the carrier signal X (for example a 1 kHz triangular wave) and, when er* greater than X, turns element S1 ON (S4 is OFF) and when er* less than X turns element S4 ON (S1 is OFF). As a result, as the R phase voltage VR of the converter there is generated a voltage proportional to the voltage instruction value er*.
Regarding the input current Ir of the R phase, when Ir* greater than Ir, the voltage instruction value er* has a negative value and Ir is increased. Contrariwise, when Ir* less than Ir, the voltage instruction value er* has a positive value and Ir is decreased. In this way, control is performed such that Ir*=Ir. Control is performed in the same way in respect of currents Is and It of the S phase and T phase.
The voltage Vd applied to DC smoothing capacitor Cd is controlled as follows. Specifically, if Vd* greater than Vd, the amplitude instruction value Ism of the input current is increased. The current instruction value of each phase is in phase with the power source voltage so active power Ps proportional to the current Ism is supplied from the AC power source SUP to the DC smoothing capacitor Cd. As a result, the voltage Vd rises and is controlled such that Vd*=Vd. Contrariwise, if Vd* less than Vd, the amplitude instruction value Ism of the input current has a negative value and power Ps is regenerated on the AC power source side. Consequently, the accumulated energy of DC smoothing capacitor Cd is reduced, lowering the voltage Vd and thereby achieving control such that Vd*=Vd.
The VVVF (variable voltage variable frequency) inverter INV and AC motor M are loads whose voltage source is the DC smoothing capacitor Cd; thus during power running operation (motoring operation) they act in a direction such as to consume the accumulated energy of capacitor Cd and to lower voltage Vd. Also, during regeneration operation, this regenerated energy is returned to smoothing capacitor Cd, so they act in a direction such as to raise voltage Vd. Since, as described above, control is performed by the PWM converter CNV such that the DC voltage Vd is constant, matching active power is automatically supplied from the AC power source during power running operation and, during regenerative operation, active power matching the regenerated energy is regenerated on the AC power source side.
Thus, with the conventional PWM converter, the DC voltage Vd can be stabilized and power regeneration achieved, enabling the problem of absence of regeneration in an electric railway DC power-generation system to be solved.
However, a PWM converter has the drawback of considerable switching loss of the switching elements, due to switching being performed at high frequency. Also, the switching elements need to have the ability to interrupt the maximum value of the AC input current, constituting the interruption current. There was therefore the problem that they had to be designed so as to be capable of withstanding interruption current even in the case of overloading for a short time (for example 300% of the rated current); the power converter therefore had to be of large size, resulting in an uneconomic system.
Thus, as described above, although self-commutated converters (called PWM converters) using pulse width modulation control were available as power converters capable of power regeneration, they were subject to the drawbacks that their cost was higher than that of diode rectifiers and that they could not tolerate very large overloading. Also, they had the problems of poor conversion efficiency and the like, owing to the large switching losses involved in PWM control.
Accordingly, one object of the present invention is to provide a novel power conversion device which is economic and of high converter efficiency, which is capable of power regeneration and which is of excellent ability to withstand overloading.
The above object of the present invention is achieved by the following construction. Specifically, a power conversion device according to the present invention comprises:
a power diode rectifier whose AC terminal is connected through an AC reactor with an AC power source;
a voltage-type self-commutated power converter whose AC terminal is connected with the AC terminal of this power diode rectifier through a recovery current suppression reactor; and
a DC smoothing capacitor connected between DC common terminals of this voltage type self-commutated power converter and the power diode rectifier, with a load device connected in parallel therewith.
With this construction, the interruption current of the voltage type self-commutated power converter can be restricted to a low level by exercising control such that, during power running operation, most of the current flows in the power diode rectifier. The input current is controlled by controlling the phase angle with respect to the power source voltage with a fixed pulse pattern (single pulse, 3-pulse, 5-pulse etc) synchronized with the power source voltage, so that the voltage-type self-commutated power converter is always operated in the vicinity of the input power factor=1. Consequently, the switching of the self-turn-off elements that constitute the self-commutated power converter is performed in the vicinity of the zero point of the input current, thereby making it possible to keep the interruption current of the elements small.
The recovery current suppression reactor performs the function of suppressing inflow of excess recovery current into the diodes of the power diode rectifier when the self-turn-off elements of the voltage type self-commutated power converter are turned ON. Usually, this reactor has an inductance value of a few tens of xcexcH i.e. it may be about two orders of magnitude smaller than the AC reactor.
On the other hand, during regeneration operation, most of the current flows in the self-turn-off elements of the voltage type self-commutated power converter. With the device of the present invention, an economic mode of use can be achieved in that for example overloading of 300% is permitted during power running operation and of 100% during regenerative operation. This mode of use is appropriate in that, in an electric railway, even if one train is performing regenerative braking, other trains are usually performing power running. When operating with 100% of regenerated power, likewise, most of the current flows in the self-turn-off elements. However, by controlling the power source power factor to practically 1 even during regenerative operation, switching of the self-turn-off elements is arranged to be performed in the vicinity of the current zero-point. Switching losses are thereby greatly reduced, making it possible to construct the self-turn-off power converter CNV of self-turn-off elements of small interruption current and thereby enabling an economic device to be provided.